AJ,
You can use offset mirror tokens to achieve this. Pick your initial
tokens for DC1N1 and DC1N2 as if they were the only nodes in your
cluster. Now increment each by 1 and use them as the tokens for DC2N1
and DC2N2. This will give you a complete keyspace within each data
center with even distribution between nodes.
If you want a more detailed description, there is a recipe for this
titled "Calculating Ideal Initial Tokens for use with Network Topology
Strategy and Random Partitioner" in the last part of the sample
chapter of "Cassandra High Performance" book
http://www.packtpub.com/cassandra-apache-high-performance-cookbook/book
-Eric